DONETSK, Ukraine — The Ukrainian army said Tuesday that it had evicted armed separatists from the international airport in Donetsk after a 24-hour gun battle, but the government in Kiev warned of a new threat as truckloads of armed Russian volunteers reportedly crossed the border.

Donetsk Mayor Oleksandr Lukyanchenko said 48 people were killed, including two civilians, in the fighting at Sergei Prokofiev International Airport.

The pro-Russian rebels said they had suffered more than 50 fatalities, many of them the result of an army attack on a truck evacuating wounded. A government spokesman said the incident was under investigation.

Shots still were being fired near the airport Tuesday afternoon, and it wasn’t clear when the facility would reopen. If the Ukraine military has cleared the facility of insurgents, it would mark a rare and swift success for a force that repeatedly has failed to dislodge separatists from city halls and police stations in eastern Ukraine.

The unity of Ukraine is riding on how the government handles the separatist uprising in the east, the latest installment of which began at 3 a.m. Monday, just hours after the conclusion of national elections that installed candy billionaire Petro Poroshenko as president. Dozens of armed insurgents of the self-styled Donetsk People’s Republic stormed the airport terminal, closed it to passenger traffic and then sent in a truckload of reinforcements.

The military waited 10 hours to respond, then flew combat jets over the scene before mounting a helicopter assault and air attacks against the insurgents.

No details of the raids have been released, including government casualties, but according to an initial account, only one government soldier was wounded. Also unknown is the fate and whereabouts of the 200 or more insurgents who took part in the assault.

U.S. Ambassador Geoffrey Pyatt said Tuesday that the militants who took part in the attack on the airport had come across the border from Russia.

Donetsk was tense Tuesday, and most residents heeded Lukyanchenko’s warning to stay indoors. Many residents expected further clashes, particularly if the military decides to attack the Donetsk regional administration building, which the Donetsk People’s Republic now occupies and uses as its headquarters.

“We have posed another ultimatum to them, and if they do not surrender, we will strike them with special weapons,” Vladislav Seleznev, a government spokesman, told reporters in Kiev. He didn’t say whether the army had acquired precision-guided munitions or some other weapons system.

Equally ominous was the possibility that more armed volunteers will head to this city of 1 million for the showdown. Just hours after a Monday appeal for help to Russian President Vladimir Putin by Dennis Pushlin, the self-appointed chairman of the “Supreme Soviet” of the Donetsk People’s Republic, armed Russian volunteers were reported to have crossed into Ukraine.

In the Luhansk region, whose People’s Republic is linked to the Donetsk People’s Republic through a union called Novorossiya, Ukrainian border guards intercepted several carloads of militants who attempted to cross illegally from Russia with a stash of assault rifles, rocket-propelled grenades and explosives. One gunmen was captured but several escaped, the state border service said.

At another location, an enormous number of armed militants appear to have crossed in early Tuesday. The Ukraine Foreign Ministry said it was protesting Russia’s failure to take action against 40 truckloads of militants who entered near Astakhovo, also in the Luhansk region.

“There are grounds to believe that Russian terrorists are being sent onto Ukrainian territory, organized and financed under the direct control of the Kremlin and Russian special services,” the ministry said.

“In fact, we are dealing with undisguised aggression against Ukraine from Russia, the Russian export of terrorism on the territory of our country,” the ministry said, asking the international community to take “immediate and decisive action” to stop Russian “aggression.”

McClatchy independently confirmed that a large number of militants had crossed into Ukraine from Russia. A Ukrainian official who closely follows issues along the border said hundreds of Russian volunteers had entered Ukraine. The official could not be quoted by name because he is not authorized to speak to the media.

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McClatchy special correspondent Kira Zheleznyak contributed to this report

Map of Ukraine locating the eastern city of Donetsk; Ukraine's interior minister says the military is now in control of the city's airport, a day after pro-Russian separatists were defeated in the national election. MCT